How do you know when to use 'mal-'?
From what I can tell, 'mal-' is a prefix that can give an adjective a negative or opposite meaning, eg. bela "beautiful" -> malbela "ugly". But can 'mal-' only be used with certain adjectives, or can it be added to anything?
Like in English, the prefixes 'un-', 'in-', etc. can only be used with certain words; is it the same sort of thing here?
And if 'mal-' can be added to any adjective, could you (for example) attach it to the word "ugly" to then mean "beautiful"? So, bela "beautiful" -> malbela "ugly", and [ugly] "ugly" -> mal[ugly] "beautiful". (I guess what I mean here is, can 'mal-' be used both ways?)
Sorry if these are really obvious questions, I'm completely new to Esperanto. And I'm not good at explaining things (especially not when it's gone midnight lol) so sorry if this is unclear.
I find the book "Being Colloquial in Esperanto" by David K. Jordan to be very useful for answering these types of questions. The book is available online: http://pages.ucsd.edu/~dkjordan/eo/colloq/colloq.html.
In particular, checkout the explanation under the heading "mal" here: http://pages.ucsd.edu/~dkjordan/eo/colloq/colloq130.html#sec13-1
So I am fairly new to Esperanto as well. But I do know that in addition to the "mal-" words (malvarma, malbela, malbona, etc) there are also a few cases where Esperanto has two separate words to talk about opposite concepts. For example war is "milito" and peace is "paco". Those are the words you'll find in the dictionary.
However, unlike most languages, you could also use "malpaco" and "malmilito" to communicate the same idea of war and peace. Even if they aren't necessarily the "proper words", their meaning is clear and Esperantists will understand you.
So to answer your questions, yes I believe you can add mal- to any adjective and use it "both ways" as you say it. That is the great power of the mal- prefix. :)
Oh, dankon! It's good to know I can "replace" adjectives with the 'mal-' forms, I'm absolutely awful at learning vocabulary so at least I'll have something to fall back on, haha.
That's the idea in Esperanto to cut down on vocabulary. There is a lot more of this going on you'll find so you only have to learn a fraction of the words your have to learn for another language.
Exactly. English requires people to learn sooooo much vocabulary. To imply the word “not” in an English word, there are the prefixes: un-, non-, dis-, dif-, mal-, anti-, de-, in-, im-, -il, -ir, a-, an-, and some other prefixes that I haven't mentioned. It is redundant to say the least.
You can use mal- on everything. I am not joking. Malmanĝi, malpost, malmatene, mal"saluton". If it follows the rules, it is correct, even if the word you created does not make any sense (yet). Ekzemple, mi scias, ke malbaleno estas la malo de baleno. A good Esperantist should be able to understand a word you created in the go. Duolingo is sadly not one of them because of the limitation of its code. Be creative.
Yes, malbaleno is an "opposite-whale," or presumably an "opposite of a whale." This might be a clever or poetic usage in the right context. However, I would probably use "malo de baleno", to be clear. Why? Because when people get carried away with mal- they see opposites where, strictly, none exist. Is red the opposite of green because they are on opposite sides of a color wheel? Malverda! Is infinity the opposite of zero? Creativity is highly encouraged, but remember that what works in your head may not be obvious to others. If you want to be understood in everyday speech, you should be somewhat conservative with mal-.
Ok, so as far as I've seen you can stick mal- onto just about any adjective to give it the opposite meaning. For your second part, because of the way Esperanto is constructed there's only one root for adjective pairs, bela and malbela in your example, so cases where you'd be able to go backwards and forwards are very unlikely exceptions rather than common cases.
Some do have separate roots for opposite in poetry, e.g. olda = maljuna; frida = malvarma (the derived "fridujo" = fridge is even used outside poetry), liva/lefta = maldekstra.
Some people use these words, but they are not sanctioned by the Akademio de Esperanto. So if you intend to use EO as an interlingvo, those words are more or less deprecated. I realize there is a school of thought that people should just do whatever they please without regard for the principles (of simplicity and limited redundancy) the language is built on. Or that certain words have become common enough, no matter how superfluous they are, to be used freely. But even if that is a valid view, I don't think new learners should be encouraged to use them without an explanation of their status. Not saying there is anything wrong with using them, but it is a good moment to explain how Esperanto is "guided" and how each user should approach his responsibility for keeping it intelligible.
Personally, I find the mal- prefix as it was defined originally one of the least amicable traits of esperanto. I am not against its presence and even its frequency, but intuitively people from nearly all languages on earth, even oriental and semitic languages, associate the prefix mal with deprivation or moral negativity. The mal- prefix doesn't suggest any form of positive opposition. About two thirds of esperanto root words come from latin or romance languages where mal means bad or evil, even in Germanic languages which come second the same prefix is instantly and instinctively felt as meaning something wrong, in Russian it means too little of something and suggests deprivation. People in Esperanto do like to use this prefix but nearly always in an emotionally negative meaning and quite never only as direct opposite of everything. When it comes to using malbona, malbela, it's perfectly OK for them to mean bad and ugly. But when it comes to malmola, hard (un-soft), people always prefer to say dura even though it is in principle an imported word, malmola is used only when softness is expected as normal and hardness perceived as a defect, a discomfort. When it comes to malvarma (cold = unwarm) it seems to be used only when warmth is expected as normal as in an apartment that should be heated and is not. Malaperi as in Gerda malaperis (Gerd has disappeared) is perfectly OK since in general a disappearance is an unhappy or unruly event. But when it comes to malfermi (opening = unclose) it seems to make sense only when the thing to open is normally expected to be closed, like certain doors or windows, and the opening is considered to be accidental. Most esperanto authors, when it comes to opening in a positive sense, rather use aperti, to make open in the sense of gaining space to breathe and pass. You would never say Lia menso malfermitas so as to mean one is open-minded, because that implies one's mind to be normally closed or to have been closed before being opened, or to have normally to be closed to adverse ideas. Pure contrariness should be expressed by other prefixes. Simple ma would mean neutral or desirable opposition, as it does in many languages especially Greek (classical) and Arabic and others, so as to denote an effort of resistance. Mavarma loĝejo would mean a cool, air-conditioned or ventilated place (protected from heat) rather than a non-heated one as malvarma would mean. Mae would mean but, nevertheless, on the other hand, (and would be easier to learn and read than the over-latinate tamen which left no trace in modern latin languages, as it looks like italian ma, French mais...) instead of male which most authors use to mean "on the down side" most of the times. Absolute geometrical opposition is musically ugly and linguistically ugly, good musicians don't use an augmented fourth as an interval of regular use as it forms no harmonic relationship, except in very specific styles of music like avant-garde jazz where disruption rather than harmony is the effect intended. The human mind doesn't like absolute oppositions where no side is more regularly the right one than the other, that's a view of things only very special minds can handle like the advaita philosophers. In music the most partially oppositional intervals used are the fifth which controls and tempers the dominant like a drain of energy from without (which would be the meaning of the prefix mal-) and the fourth which is rather a positive effort to meet an obstacle victoriously (the meaning of ma-). Undoing a thing by the very inverse process it has been built by man or nature should be meant by yet another prefix like German ent-, whereas symmetrically opposing something in return or in reply should be meant by the Greek prefix anti- or ant as is already used in the general political language. Fari = to do. Malfari = to undo in the sense of demolishing, destroying, generally out of spite or out of neglect Mafari = to reverse a damage, to make the place clean of a bad action or untimely happening Entfari, to disassemble, Antifari, to make something in response, like a monument for a contrary cause, like a Russian pavilion facing an American one in an exhibition. Varma = warm or hot, depending ; malvarma = cold, for lack of heat or heating ; mavarma, cold or cool, by protection against heat like a shade ; entvarma, made cold or cool by intelligent reverse action = refrigerated or air-conditioned ; antivarma = opposing coldness to excessive warmth, or being cold by principle.